Add initial backend API implementation using existing functions to replicate the same function calls needed in existing streamlit app

FastAPI Docs and Playground!

Added the following routes

POST /upload/{filetype}

• filetype can be voter_records or petition_signatures
• voter records is stored in app state
• petition signatures is stored in local temp directory

POST /ocr

• Loads all data and runs the OCR pipeline
• Returns csv response of results

DELETE /clear

• Clears out all the data that was loaded

Updated README on instructions to run and also fix some minor formatting issues